Detergent compositions, such as fabric washing compositions and machine dishwashing compositions may be provided in the form of granular compositions or as tablets compacted from granular compositions. Such detergent compositions typically comprise a builder in order to improve detergency and to reduce the negative effects of hardness ions such as calcium or magnesium from hard water which may be used during the washing process. For ecological and regulatory reasons, the use of phosphates as builders is now highly undesirable, and in some countries, and under certain circumstances, is forbidden.
Chelating agents suitable for use as replacements for phosphate include compounds such as MGDA salts. These have the benefit of being both biodegradable and obtainable from renewable sources. MGDA is methylglycine diacetic acid, which in its acid form has three acid protons which may be replaced by other cations to form salts.
MGDA salts are usually provided commercially as an aqueous solution having an active (i.e. anhydrous solid) content from 35 to 50% by weight of MGDA salts. Usually, the trisodium salt is used (referred to in the art as MGDA Na3) and such material is widely available commercially, from companies of the BASF group.
In order to use MGDA salts as a builder or chelant in granular detergent compositions, the aqueous solution of MGDA salt may be dried in order to form builder particles or granules in solid form. However, such granules have been found, in the prior art, to exhibit extreme hygroscopic behaviour which is undesirable in granular detergent compositions. Such hygroscopic behaviour, characterised by uptake of moisture from the ambient surroundings, may render granules cohesive, leading to storage, handling and manufacturing difficulties arising from poor flow properties. The formulations containing such granules may even set or solidify into a solid mass when stored at high humidity. In particular, it is well established from the prior art that MGDA powder produced by conventional spray-drying of aqueous MGDA solution has unacceptable caking behaviour when subject to water uptake as a result of hygroscopicity of the powder.
Furthermore, when such builder granules are incorporated into detergent tablets, typically formed by compaction of a variety of granules of differing types, the presence of hygroscopic granules may have an adverse effect upon the disintegration characteristics, dissolution behaviour and cohesiveness of the resulting tablet, following storage. Redistribution of moisture from other granular ingredients to the builder granules, or uptake of moisture from the ambient atmosphere, may lead to changes in the characteristics of tablets over time and may lead to granules being cemented together in the tablet in an undesirable fashion, leading to reduction in disintegration and dissolution rate for such tablets in use.
The organic nature of builders, such as MGDA salts may also lead to their presence in formulations having an adverse effect on explosion risks in factories where the material is handled, when they are present at an effective level as builder in compositions. The high organic content of dust in the atmosphere derived from attrition arising during processing of such builder granules may result in ignition or explosion risk. High levels of organic material may also give rise to storage problems arising from self-heating of warm granules, when they are held in hoppers or storage silos, arising from spontaneous auto-oxidation at elevated temperatures, potentially giving rise to dangerous, run-away self-heating effects. Individual granules to be stored in bulk should be free from risk of self-heating when subjected to high ambient temperatures.
EP 1803801, WO 2006/002954, and GB 2415695 describe the use of polymeric coating materials on builder granules including MGDA salts, applied to tackle the problems of hygroscopicity for such chelating agents when used as detergent builders.
WO 2010/076291 discloses coated particles containing chelating agents such as MGDA salts wherein the coating agent is a scale-inhibiting additive-containing material.
U.S. Pat. No. 5,981,798 discloses a crystalline solid with low hygroscopicity which essentially consists of MGDA derivatives (salts) and which is prepared by adjusting the water content of a starting material containing the MGDA derivatives to a value from 10 to 30% by weight based upon the starting material then subsequently bringing about crystallisation of the solid MGDA derivative. This publication also states “Spray-drying processes (e.g. in a spray tower or spray fluidized bed) by contrast result in an amorphous solid. In this form, the solid is often highly hygroscopic and, on open storage under ambient conditions, its. flowability is lost within a short time.”
WO 2010/133618 discloses a process for preparing a powder containing MGDA derivatives, such as the sodium salt of MGDA, wherein the resulting product comprises a powder having MGDA salt said to have a degree of crystallinity greater than or equal to 30%. The process disclosed requires a starting material which is an aqueous solution comprising the MGDA salt in a concentration range from 20 to 60% based on the total weight of the aqueous solution. In a first process step, the aqueous solution is concentrated in an evaporator with rotating internals which are arranged a distance from the inner wall of the evaporator of less than or equal to 1% of the diameter of the evaporator to give a crystal slurry having a solid concentration in the range from 60 to 85% by weight, based on a total weight of the crystal slurry. In a second process step, the crystal slurry is allowed to mature in a paste bunker and then in a thin-film contact dryer, wherein the total dwell time in the paste bunker and in the thin-film contact dryer is greater than or equal to 15 minutes. This publication also explains that powders are produced industrially in spray-drying plants, which leads to solids with high amorphous fractions. It is stated that this leads to highly hygroscopic behaviour and poor storability and further processability (for instance in tabletting presses). The publication discloses two MGDA Na3 crystalline states (referred to in this present specification as monohydrate and as dihydrate) obtained by the process disclosed in WO 2010/133618.
WO 2009/103822 discloses a process for the preparation of free-flowing granules of low hygroscopicity containing one or more MGDA salts. The process comprises the steps of heating a concentrated slurry comprising the MGDA salts to a temperature in the range 50 to 120° C., wherein the slurry has a solid content in the range of 45 to 70%. The slurry is then spray-granulated in a spray-granulator using an air inlet temperature of 120° C. or less. The document explains that in spray-granulation, it is the objective to spray the slurry onto existing seeds in the drying chamber and to dry the slurry at that location so the seeds grow into granules. Only when the granules reach a certain particle size is the product discharged from the equipment. Such spray-granulation equipment is not conventional granulation equipment used traditionally in the detergent industry, and typically operates at lower production rates than equivalent conventional spray-drying equipment. In its introduction, this publication states: “Particles of MGDA made via conventional spray drying processes tend to be fine and dusty, have a high tendency to absorb water at ambient conditions and loose their free-flowiness. The resulting products are hygroscopic, resulting in sticky powder and even in lumps.”